Check the plenum chambers - look under the bonnet below the windscreen - there is one or often two chambers under the screen at either side of the engine bay.  They have drains in the bottom to get rid of water but these often block with dead leaves, dirt etc.  The chambers should be dry - if not, the drains are blocked.  Poke around to find the drain and clear the blockage to let the water drain away, then keep them clear by checking regularly.  If not this, check the pollen filter cover - often not refitted correctly.

Tim